There is a conventionally known two-stroke engine having a scavenging port for introduction of a working gas containing fuel into a combustion chamber, and an exhaust port for discharge of gas in the combustion chamber therefrom, wherein a piston connected to a crankshaft through a connecting rod reciprocates in the combustion chamber with rotary drive of the crankshaft, to control opening and closing of the scavenging port and the exhaust port, wherein as the piston moves from a top dead center (TDC) to a bottom dead center (BDC), the exhaust port and the scavenging port open in the order named, and wherein as the piston moves from BDC to TDC, the scavenging port and the exhaust port close in the order named.
Incidentally, in the foregoing conventional two-stroke engine there is almost no pressure difference between the combustion chamber and the exhaust port during a period of time after an end of a scavenging stroke in which the piston moves from TDC to BDC, and before a start of a compression stroke in which the scavenging port closes and the exhaust port also closes soon thereafter with an ascent of the piston, and it causes fresh gas (air-fuel mixture) in the combustion chamber to easily flow out through the exhaust port. The outflow results in so-called “blow-by,” i.e., the whole fresh gas introduced through the scavenging port into the combustion chamber flows out directly through the exhaust port without staying inside the combustion chamber. This blow-by fresh gas was released as unburnt gas into the atmosphere without being purged. Since the unburnt gas consists primarily of fuel, it thus contains a large amount of hydrocarbons and increases a total hydrocarbon (THC) amount in exhaust gas.
For avoiding it, for example, the two-stroke engine described in Patent Document 1 below is constructed in a configuration wherein before introduction of fresh gas in the scavenging stroke, exhaust gas for blow-by is first introduced through a first scavenging port into the combustion chamber and the fresh gas is then introduced through a second scavenging port to scavenge the combustion chamber, thereby preventing the blow-by of fresh gas and reducing the THC amount in the exhaust gas.